The invention relates to an optical communications system known from: C. R. Giles, "Propagation of Signal and Noise in Concatenated Erbium-Doped Fiber Optical Amplifiers", Journal of Lightwave Technology, Vol. 9, No. 2, Feb., 1991, pages 147-154, particularly FIG. 2.
C. R. Giles, "Modeling Erbium-Doped Fiber Amplifiers", Journal of Lightwave Technology, Vol. 9, No. 2, Feb., 1991, pages 271-283, particularly FIG. 2 at page 272, discloses that the gain of a fiber-optic amplifier is a function of the wavelength of its optical input signal, that is, the emission wavelength of the optical transmitter of the transmission system. The wavelength dependence of the gain (dashed curve) is shown in two examples for different compositions of Er.sup.3+ doped pieces of light waveguides which are employed with preference in fiber-optic amplifiers. In addition, the wavelength dependence of the absorption in the same light waveguide pieces is shown (solidly drawn curve). It can be seen that the gain curve and the absorption curve have their maximum at the same wavelength and that this wavelength is almost the same for the various types of light waveguide pieces.
In optical communications systems employing such fiber-optic amplifiers, the described wavelength dependence of the gain brings about the following problem: if the emission wavelength of the semiconductor laser employed in the optical transmitter does not lie at the wavelength at which the fiber-optic amplifier has its maximum gain, but only slightly next to it, the gain is considerably poorer. Moreover, if several fiber-optic amplifiers are arranged in series, noise at the wavelength of maximum gain may accumulate and thus worsen the signal to noise ratio.
Added to this is also the following problem: is a so-called "laser chirp" is known in Optical transmission systems, that is, an undesirable fluctuation in the wavelength of the optical signal that is a function of signal amplitude of an electrical signal to be transmitted optically. If the wavelength of the transmitted optical signal is not the wavelength that is optimum with respect to gain in fiber-optic amplifiers, the degree (steepness) with which the gain curve depends on the wavelength is very great and the undesirable wavelength fluctuations present in the optical signal are converted by the optical gain into considerable, undesirable amplitude fluctuations. In the region of the maximum of the gain curve such amplitude fluctuations caused by wavelength fluctuations are noticeably less.
Consequently it would be desirable for the emission wavelength of the optical transmitter of the transmission system to be equal to that wavelength at which the particular type of fiber-optic amplifier, of which one or several are included in the light waveguide path, has its maximum gain. If one considers a chain of fiber-optic amplifiers connected in series in a transmission path, each having the described wavelength dependence of its gain, it is clear that the fiber-optic amplifiers arranged in the chain as a whole produce a good amplification of the transmitted optical signals only in a narrow band of wavelengths. Although semiconductor lasers are available on the market whose emission wavelengths lie at the optimum wavelength (e.g., 1536 nm or 1555 nm) with respect to the fiber-optic amplifiers, there are deviations due to fluctuations between different individual units of a certain type of laser and due to changes in the emission wavelength as a result of aging of the semiconductor lasers, as well as due to fluctuations in respective ambient temperatures.
The cited publications do not describe how it can be accomplished that the emission wavelength of the optical transmitter of the system is adapted to the wavelength at which the light waveguide path including one or several fiber-optic amplifiers produces an effective gain.